1. Field of the Invention
The present invention relates to an Rxe2x80x94Txe2x80x94B system rare earth permanent magnet containing, as main components, R (wherein R represents one or more rare earth elements, providing that the rare earth elements include Y), T (wherein T represents at least one transition metal element essentially containing Fe, or Fe and Co), and B (boron).
2. Description of the Related Art
Among rare earth permanent magnets, an Rxe2x80x94Txe2x80x94B system rare earth permanent magnet has been increasingly demanded year by year for the reasons that its magnetic properties are excellent and that its main component Nd is abundant as a source and relatively inexpensive.
Research and development directed towards the improvement of the magnetic properties of the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet have intensively progressed. For example, Japanese Patent Laid-Open No. 1-219143 discloses that the addition of 0.02 to 0.5 at % of Cu improves magnetic properties of the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet as well as heat treatment conditions. However, the method described in Japanese Patent Laid-Open No. 1-219143 is insufficient to obtain high magnetic properties required of a high performance magnet, such as a high coercive force (HcJ) and a high residual magnetic flux density (Br).
The magnetic properties of an Rxe2x80x94Txe2x80x94B system rare earth permanent magnet obtained by sintering depend on the sintering temperature. On the other hand, it is difficult to equalize the heating temperature throughout all parts of a sintering furnace in the scale of industrial manufacturing. Thus, the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet is required to obtain desired magnetic properties even when the sintering temperature is changed. A temperature range in which desired magnetic properties can be obtained is referred to as a suitable sintering temperature range herein.
In order to obtain a higher-performance Rxe2x80x94Txe2x80x94B system rare earth permanent magnet, it is necessary to decrease the amount of oxygen contained in alloys. However, if the amount of oxygen contained in the alloys is decreased, abnormal grain growth is likely to occure in a sintering process, resulting in a decrease in a squareness. This is because oxides formed by oxygen contained in the alloys inhibit the grain growth.
Thus, a method of adding a new element to the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet containing Cu has been studied as means for improving the magnetic properties. Japanese Patent Laid-Open No. 2000-234151 discloses the addition of Zr and/or Cr to obtain a high coercive force and a high residual magnetic flux density.
Likewise, Japanese Patent Laid-Open No. 2002-75717 discloses a method of uniformly dispersing a fine ZrB compound, NbB compound or HfB compound (hereinafter referred to as an Mxe2x80x94B compound) into an Rxe2x80x94Txe2x80x94B system rare earth permanent magnet containing Zr, Nb or Hf as well as Co, Al and Cu, followed by precipitation, so as to inhibit the grain growth in a sintering process and to improve magnetic properties and the suitable sintering temperature range.
According to Japanese Patent Laid-Open No. 2002-75717, the suitable sintering temperature range is extended by the dispersion and precipitation of the Mxe2x80x94B compound. However, in Example 3-1 described in the above publication, the suitable sintering temperature range is narrow, such as approximately 20xc2x0 C. Accordingly, to obtain high magnetic properties using a mass-production furnace or the like, it is desired to further extend the suitable sintering temperature range. Moreover, in order to obtain a sufficiently wide suitable sintering temperature range, it is effective to increase the additive amount of Zr. However, as the additive amount of Zr increases, the residual magnetic flux density decreases, and thus, high magnetic properties of interest cannot be obtained.
Hence, it is an object of the present invention to provide an Rxe2x80x94Txe2x80x94B system rare earth permanent magnet, which enables to inhibit the grain growth, while keeping a decrease in magnetic properties to a minimum, and also enables to further improve the suitable sintering temperature range.
The present inventor has found that when a product that is rich in Zr exists in an R2T14B phase constituting the main phase of an Rxe2x80x94Txe2x80x94B system rare earth permanent magnet, the permanent magnet enables to inhibit the grain growth, while keeping a decrease in magnetic properties to a minimum, and to improve the suitable sintering temperature range. That is to say, the present invention provides an Rxe2x80x94Txe2x80x94B system rare earth permanent magnet, which is a sintered body comprising a main phase consisting of an R2T14B phase (wherein R represents one or more rare earth elements (providing that the rare earth elements include Y), and T represents one or more transition metal elements essentially containing Fe, or Fe and Co), and a grain boundary phase containing a higher amount of R than the above main phase, wherein a product that is rich in Zr exists in the above R2T14B phase.
In the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet of the present invention, the product that is rich in Zr has a platy or acicular form.
In the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet of the present invention, the amount of oxygen contained in the above sintered body is preferably 2,000 ppm or less. This is because effects obtained by the presence of the Zr rich product in the R2T14B phase, such as the inhibition of the grain growth or the extension of the suitable sintering temperature range become significant, when the amount of oxygen contained in the sintered body is as low as 2,000 ppm or less.
In the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet of the present invention, the sintered body preferably has a composition consisting essentially of 28% to 33% by weight of R, 0.5% to 1.5% by weight of B, 0.03% to 0.3% by weight of Al, 0.3% or less by weight (excluding 0) of Cu, 0.05% to 0.2% by weight of Zr, 4% or less by weight (excluding 0) of Co, and the balance substantially being Fe.
In the Rxe2x80x94Txe2x80x94B system rare earth permanent magnet of the present invention, Zr is contained in the sintered body, more preferably within the range between 0.1% and 0.15% by weight.